Modern tank armor is not of uniform thickness around the vehicle, meaning that an MBT’s sides and rear are far less protected than the front. Ingame, this is reflected by Directional Armor, but there is a quirk to this. Planetside 2’s damage model for tanks is not dependent on where exactly you land your hit, but rather where you take your shot relative to the position of the other vehicle. Because of this, it’s possible to deal critical damage by shooting an enemy tank in the gun barrel, provided you’re attacking from a rear angle. This is demonstrated in the following image:
Here you can see a tank divided into four quadrants. The front armor of a tank has a 0% damage modifier, meaning incoming damage does 100% of its direct damage value listed ingame.
The sides of a tank have a -15% damage resistance, meaning a hit in this angle does 115% of base direct damage value.
The rear of a tank has a -50% damage resistance, meaning a hit from the rear arc does 150% of the base direct damage value.
The damage calculation is further complicated by the categorization of weapons into various resist categories, such as Tank Cannons or Air to Ground Warheads. These categories exist to reduce situations of a weapon doing extreme damage against specific target classes while being terrible at damaging other vehicle types- here a blanket nerf would cripple the weapon, but the resist table allows for fine-tuning of damage output.
As a demonstration, here is a hypothetical scenario. A vanguard with the Titan-150 AP main gun attacks a Magrider from the side.
Titan 150 AP has a direct damage value of 850.
MBTs have an innate -50% damage resistance to tank shells.
MBTs have a directional damage resistance of -15% to side damage.
Magriders have 5000 hit points.
First we apply the innate resistance of -50%, which translates to a shell doing 50% more damage.
Next we apply the directional modifier of -15%, which means 15% more damage is dealt.
1275*1.15=1466 final damage.
A baseline resist value of 100% means the weapon will deal zero damage. Infantry have a baseline resistance of 0% to all weapons.
You can find a table of all resistances here: https://planetside.fandom.com/wiki/Vehicle_armor_and_damage_resistance